Mass transit in the Northeast was hit hard by Sandy. New Yorkers had to do without the subway, for example, but now the transit system has mostly recovered. It’s a perfect time to be thankful for the extensive network of public transportation that exists in the city. In fact, it means that New Yorkers have one of the smallest per-capita carbon footprints in the U.S.

But you’re probably sensing a hidden side here, right?

"Mass transit can be an incredible boon for the environment," says Eric Morris, a regular contributor to Freakonomics and a professor of urban planning at Clemson University. He told Freakonomics' Stephen Dubner: "It can also not help the environment or maybe even hurt the environment."

Wait. What?

“Obviously, the energy expenditure in moving around a transit vehicle per passenger mile depends on the number of passengers," Morris continues. "Whether you have one passenger in a bus or 40 passengers in a bus, you're going to be expending almost the same amount of energy. So it all depends on the ridership and the occupancy that transit vehicles and, for that matter, autos carry."

So here comes the rub. The average American car carries 1.6 people -- not many, of course, when you're comparing it to mass transit. On the other hand, the average bus carries only 10 people. And a bus burns a lot more fuel than a car. Not exactly what mass-transit advocates would have us believe.

Which led Morris to this rather surprising conclusion: "Typically, moving a passenger a mile by bus requires roughly 20 percent more energy than moving a passenger around by car…So, just in terms of energy expenditure, bus actually fares worse than car."

And trains? Trains are on average better than cars, roughly two-thirds of energy per passenger -- although that number is warped a bit by the New York City subway, which is just a monster of efficiency.

The caveat here: it’s hard to make an apples-to-apples comparison, since transportation is so complex. But here's Eric Morris's point: In terms of energy efficiency, mass transit is not the panacea that a lot of people would like to think. Yes, it works great in a dense urban area like New York, but Morris argues that we've already picked a lot of that low-hanging fruit, and that light-rail systems in places like Cleveland, Pittsburgh and Memphis actually do worse than cars in terms of energy efficiency, simply because they're underused.

"In general, pumping up ridership by constructing new transit systems or adding new transit service has to be looked at very skeptically," he says. "On the other hand, if we can persuade more people to leave cars and move onto the existing transit system that we already have, that's a complete win for the environment."

There are a lot of trade-offs here that are hard to measure. A few examples: commuting time, land usage for parking, the cost of ownership of different kinds of vehicles, and the traffic fatalities that come with car travel.

So if liberals hate the idea of discouraging mass transit, here's an idea that conservatives can hate: you can compel people to leave their cars by raising tolls and gas and parking taxes to incentivize more people to ride the transit systems we've already spent billions on.